Implement time-dependent blaze correction for STIS

[stscirij]

Currently the blaze correction coefficients are stored in the PHOTTAB reference file, which has both spectrophotometric sensitivty curves for every spectroscopic mode and blaze correction coefficients for echelle modes. It has become very unwieldy to implement time-dependent blaze correction coefficients using the current reference file structure because of the need to ensure that only the items that need changing are changed and everything else is left untouched. Extracting the blaze correction coefficients from the PHOTTAB reference file and creating a new BLAZETAB reference file will simplify the process of keeping the calibration information of these two modes up to date without the risk of changes in one type of data causing problems in the other type.